Effect of Nanoclay on Thermal Conductivity and Flexural Strength of Polymethyl Methacrylate Acrylic Resin.

STATEMENT OF THE PROBLEM
The mechanical and thermal properties of polymethyl methacrylate (PMMA) acrylic resin should be improved to counterweigh its structural deficiencies.


PURPOSE
The aim of this study was to compare the flexural strength and thermal conductivity of conventional acrylic resin and acrylic resin loaded with nanoclay.


MATERIALS AND METHOD
The methacrylate monomer containing the 0.5, 1 and 2 wt% of nanoclay was placed in an ultrasonic probe and mixed with the PMMA powder. Scanning electron microscopy was used to verify homogeneous distribution of particles. Twenty-four 20×20×200-mm cubic samples were prepared for flexural strength test; 18 samples containing nanoclay and 6 samples for the control group. Another 24 cylindrical samples of 38×25 mm were prepared for thermal conductivity test. One-way ANOVA was used for statistical analysis, followed by multiple-comparison test (Scheffé's test). Statistical significance was set at p< 0.05.


RESULTS
Increasing the concentration of nanoclay incorporated into the acrylic resin samples increased thermal conductivity but decreased flexural strength (p< 0.05).


CONCLUSION
Based on the results of this study, adding nanoclay particles to PMMA improved its thermal conductivity, while it had a negative effect on the flexural strength.


Introduction
In recent years, perceiving the advances in the nanotechnology science, many attempts have been made to use these particles in dental acrylic resin to improve its fatigue behavior, impact strength, and thermal conductivity, and some successes have been achieved. [1] However, there are some discrepancies between the results of these studies. [1][2][3] Thermal conductivity is an important feature of acrylic resin which affects prosthesis acceptance by the patient. Besides, it has major impacts on salivary glands secretion (especially parotid gland) and health of denture supporting tissues. One of the main drawbacks of polymethyl methacrylate (PMMA) resin is its low thermal conductivity. [4][5] Nano titanium dioxide can be incorporated into dental materials to mimic the opacity of teeth [1] or to decrease bacterial adhesion to these materials. [2] However, studies show that it cannot improve the mechanical properties of composite resins. [6][7] In addition, nano titanium dioxide can increase the hardness of acrylic resins used as temporary crowns. [3] Nanoclay is another material which is used to improve the properties of composite and acrylic polymers. Montmorillonite (MMT) is one of the available forms of nanoclay, which consists of small layers with internal octahedral layer interposed between two tetrasilicate layers. This structure is claimed to prevent formation of cracks and, therefore, it can improve flexural strength. [8] Atai et al. [9] used PMMA-grafted nanoclay as filler for dental adhesives which increased the shear bond strength of the adhesive and improved the dispersion stability of the particles 40 folds compared to pure nanoclay.

Materials and Method
This research made use of polymethyl methacrylate (PMMA; SR Triplex Hot, Ivoclar Vivadent, Liechtenstein, Germany) as a heat-curing acrylic resin, and Cloisite 20A commercial nanoclay material with a diameter of <6 μm and a density of 1.77 g/mL (Southern clay Products Inc.; Austin, U.S.A) which was modified with quaternary ammonium salts. Nanoclay in three concentration groups at 0.5, 1 and 2 wt% were mixed with heat-curing acrylic resin.
These concentrations were determined based on a pilot study which revealed that addition of less than 2% nanoclay to the acrylic resin increases thermal conductivity with no effect on its mechanical properties. Before acrylic resin packing procedures, to achieve the best distribution quality, the monomer containing the specified wt% of nanoclay was placed in an ultrasonic probe (Hielscher Ultrasonics GmbH, UP200H, Germany) for 5 minutes; [12][13] and then mixed with the powder. After the paste achieved a doughy consistency, it was packed into steel molds.
Then, the specimens were removed from the molds after being cured.
To evaluate the thermal conductivity, two holes were produced, each 6.5 mm away from one end of the sample. [14][15] Then, the samples were placed in a heat conduction tester; two thermocouple systems were placed within these two holes. Water flew through holes and the difference in temperature between the two ends was determined by the thermometers of the tester. The thermal conduction coefficient (k) was calculated for each sample using following formula: The mean, average, and mode were calculated in each group and normal distribution curve was appraised.
One-way ANOVA and then multiple-comparison test (Scheffé's test) were used for statistical analysis. Statistical significance was set at p< 0.05.

Results
The results of One-way ANOVA showed significant differences in thermal conduction values of the groups ( Table 1). The results of multiple-comparison test for  The comparison of thermal conductivity results of all groups are illustrated in Figure 2.  In different studies, [20][21]  However, some studies reported an improvement in mechanical properties with an increase in the filler content of acrylic resins. Acosta et al. [25] showed that titanium oxide was an appropriate filler to improve the mechanical properties of acrylic resins. Solhi et al. [8] reported that incorporation of nanoclay filler reinforced with PMMA into adhesive resin improved its flexural modulus. They also observed that higher filler content increased the flexural modulus. A study by Chisholm et al. [26] showed that PMMA-grafted nanoclay filler loading could improve the flexural strength of FRCs.
Moreover, in the study of Labella et al., silane-treated hydroxyapatite powder was used with different filler contents and showed enhanced flexural strength, tensile strength and Vickers hardness. [27] An important problem with the use of metal particles including nano-silver is a change in acrylic polymer color, which limits its use in the esthetic zone. [18,[28][29] However

Conclusion
In the present study, loading of acrylic resin with modified nanoclay at 1 and 2 wt% concentrations increased the thermal conductivity; however, it significantly decreased the flexural strength of the acrylic resin.